Big chunks of history (and rock) are missing in North America, study says

This rock exposes the Great Unconformity, where 1.1 billion-year-old Pikes Peak granite is topped with 510 million-year-old sandstone outside of Manitou Springs, Colorado.

(CNN)Visitors to Pikes Peak in Colorado may not notice that anything is amiss, but ask a geologist and she'll tell you that something is missing.

At rocky sites across America, including Pikes Peak and the Grand Canyon, 550 million-year-old rock sits on top of rock that has existed as long as 3 billion years. But there's nothing in between. It's as though that time period between 3 billion years ago and 550 million years ago has literally been wiped from Earth.
It's called the Great Unconformity, and some researchers believe it's the result of one massive cataclysmic event in Earth's history.
    "Researchers have long seen this as a fundamental boundary in geologic history," said Rebecca Flowers, study author and associate professor in the University of Colorado at Boulder's department of geological sciences, in a statement.
      That boundary represents the boundary between rocks that don't contain fossils, and those that do. About 540 million years ago, the so-called Cambrian explosion occurred. This event marks where there is a sudden wealth of a diverse range of animal fossils in Earth's rocks.
      Flowers and her colleagues investigated the boundary in a study published Monday in the journal Proceedings of the National Academy of Sciences.
      At Pikes Peak, they found the clear dividing line between rocks that were less than 510 million years old and rocks that reached 1 billion years old. The older rocks are what is referred to as "basement" rock.
        Researcher Rebecca Flowers stands near a rock outcrop on Pikes Peak in Colorado.
        "There is a lot of the geological record that is missing," Flowers said. "But just because it's missing doesn't mean that this history is simple. Only recently have we had the ability to reach far enough back in time to start filling in that gap."
        The researchers used thermochronology to study rock samples. This allowed them to study atoms inside the samples that preserve the heat history of the rocks at Pikes Peak. Together, the heat history of the samples provided a timeline of when the rocks were hot or cold.
        Thermochronology enabled the researchers to pinpoint when basement rocks were exhumed at Pikes Peak. An event caused an upheaval of rock there 700 million years ago.